1. Field of the Invention
The present invention relates to a method of detecting a condensing spot position of a condenser for condensing a laser beam in a laser beam processing apparatus.
2. Description of the Related Art
In a semiconductor device manufacturing process, planned dividing lines called streets are provided in a grid pattern on a surface of a substantially circular plate-shaped semiconductor wafer to partition a plurality of regions, and devices such as ICs or LSIs are formed in the thus partitioned regions. Then, the semiconductor wafer is cut along the streets to divide the regions in which the devices are formed, thereby manufacturing the individual semiconductor devices.
As a method of dividing the semiconductor wafer or the like along the streets, a laser beam processing method has been tried in which the wafer is irradiated with a pulsed laser beam capable of being transmitted through the wafer, while adjusting the condensing point of the laser beam to the inside of the regions to be divided. In the dividing method based on the laser beam processing method, the wafer is irradiated with a pulsed laser beam with such a wavelength as to be transmitted through the wafer, from the side of a surface on one side of the wafer, while adjusting the condensing point of the laser beam to the inside of the wafer, to continuously form an altered layer inside the wafer along the streets, and an external force is exerted on the wafer along the streets where strength is lowered by the formation of the altered layer, to thereby divide the workpiece (refer to, for example, Japanese Patent No. 3408805). In the case of forming the altered layer inside the workpiece along the streets formed on the workpiece in this manner, it is important for the condensing point of the laser beam to be positioned into a position of a predetermined depth from the upper surface of the workpiece.
In addition, as a method of dividing a plate-shaped workpiece such as a semiconductor wafer, a method has been proposed in which the workpiece is irradiated with a pulsed laser beam with such a wavelength as to be absorbed in the workpiece along the streets formed on the workpiece, to form laser beam processed grooves through ablation, and the workpiece is cut up along the laser beam processed grooves by use of a mechanical breaking device (refer to, for example, Japanese Patent Laid-open No. Hei 10-305420). In the case of forming the laser beam processed grooves along the streets formed on the workpiece in this manner, also, it is important for the condensing point of the laser beam to be positioned into a position of a predetermined depth in the workpiece.